The present invention relates to a shape measuring apparatus and a shape measuring method for scanning a measuring surface by lightly contacting a stylus with the measuring surface, sequentially reading coordinates, and thereby measuring the shape of the measuring surface.
Extensive progresses in size reduction and performance improvement of manufactured products cause increasing needs for components having such complicated shapes as cannot be manufactured without measurement and components that require higher accuracy. For scanning measurement of arbitrary three-dimensional shapes of measuring objects such as those components, there have been provided shape measuring apparatuses that scan a measuring surface by lightly contacting a stylus with the measuring surface, sequentially reading coordinates, and thereby measuring a shape of the surface. Further, there have been proposed various techniques for automatically scanning the measuring surface by the stylus for such shape measurement.
For instance, Japanese Patent Application Laid-open Publication No. S57-33301 discloses a shape measuring apparatus in which strain gages are mounted on four sites of a probe shaft having a probe on an extremity thereof. The strain gages detect a direction and magnitude of a strain in the probe shaft caused by a measuring force from a measuring surface. Movements of the probe in a direction perpendicular to the detected direction of the measuring force can achieve automatic scanning measurement. Although not disclosing such a control as to maintain the quantity of the strain constant, this publication discloses that adding the quantity of the strain to coordinate measurement data reduces measurement errors.
Japanese Patent No. 3101322 discloses a shape measuring apparatus in which a measuring stylus is supported on a probe by support members employing disc-like springs so as to be capable of moving in directions of X-, Y-, and Z-axes. Positions of the measuring stylus relative to the probe in the directions of the X, Y and Z-axes are read from movement of a slit provided on an upper part of the measuring stylus projected onto a position-sensitive photodetector. Scanning by the probe is performed in a direction perpendicular to directions of measuring forces that have been detected.
Japanese Patent Application Laid-open Publication No. 2005-345123 discloses a shape measuring apparatus which achieves scanning measurement with generally constant measuring forces by performing scanning of the measuring surface at velocities obtained by adding velocity components for correcting increases and decreases not less than a given constant value in detected measuring forces to velocity components perpendicular to the detected measuring forces.
Japanese Patent Application Laid-open Publication No. 2003-240538 discloses a method for controlling scanning in which a probe is moved on extensions of straight lines respectively linking a former measurement point and a present measurement point and in which positions of the probe are shifted in a direction such that measuring forces recovers a given constant value if the measuring force exceeds a predetermined limit value.
In the shape measuring apparatuses disclosed in the above-first and second publications, the probe is moved in the direction perpendicular to that of the measuring force. However, the measuring force acting on the stylus is a resultant force of a force in a direction perpendicular to the measuring surface and of a frictional force that acts in a direction parallel to the measuring surface. Accordingly, the direction perpendicular to the measuring force is not coincident with the direction parallel to the measuring surface, but actually is a direction deviating from the measuring surface. Therefore, the methods disclosed in the above first and second publications result in that the probe deviating from the measuring surface.
In the shape measuring apparatus disclosed in the above third publication, decreases in the measuring force cause movements of the probe in directions for correcting the decreases (directions in which the probe is pushed toward the measuring surface), whereas recoveries of the measuring forces to the given constant value cause movements of the probe in directions such that distances from the measuring surface increase. These result in that the probe moves on a sinusoidal track with respect to the measuring surface, causing difficulty in performing smooth measurement.
According to the method disclosed in the above fourth publication, the probe moves straight on the extensions of straight lines linking the former and present measurement points until the measuring force acting on the stylus reaches the limit value even if the measuring surface is curved. Upon excess of the measuring force over the limit value, the probe moves in a direction perpendicular to the extension for correction. These result in that the stylus fails to move smoothly along the measuring surface and the measurement force is inconstant. Further, in case that the measurement surface forms a wall constituting an angle smaller than a right angle, the measuring force is not corrected even though the probe moves in the direction perpendicular to the extension upon detection of excess of the measuring force over the limit value. Thus, in this case, the probe needs to return an initial position and perpendicularly turn its course, resulting in unsmooth scanning measurement.
As described above, the conventional scanning measuring methods have failed to achieve and suggest smooth scanning measurement. The failure to perform smooth scanning causes vibration which increases measurement error as well as increase in measuring time.